1. Field of the Invention
The present invention relates to a semiconductor memory element that stores binary data 0 or 1 in the form of resistance. The invention relates also to a semiconductor memory device that has semiconductor memory elements of this type.
2. Description of the Related Art
Memory cells having a two-layered gate composed of a floating gate provided on a semiconductor substrate and a control gate provided on the floating gate are widely used as electrically programmable, nonvolatile semiconductor memory elements. Further, NAND-type semiconductor memory devices, each having memory cells of this type connected in series to acquire a large storage capacity, have been put to practical use. In the memory cell having a two-layered gate, however, the insulating film surrounding the floating gate cannot be as thin as desired because the floating gate must accumulate an electric charge. Consequently, this memory cell cannot be made smaller or driven at a lower voltage.
MRAMs have been proposed, which incorporate nonvolatile memory cells, such as magnetic tunnel-junction (MTJ) cells, which have no floating gates and which have their resistance changed. To read data from any memory cell that stores the data in the form of resistance, it is usually necessary to supply a current to the memory cell to detect the output voltage thereof or to apply a voltage to the memory cell to detect the output current thereof. Inevitably, the circuits peripheral to memory cells tend to become large. In view of this, a MTJ cell may be combined with a transistor in order to detect the change in resistance from the change in conductance of the transistor. (See Jpn. Pat. Appln. KOKAI Publication No. 2001-273758.)
The MTJ cell has but a low resistance-change rate, and the transistor may have a threshold value that differs from the design value. In view of these, the idea of combining the MTJ cell may be combined with a transistor cannot change the conductance of the transistor as much as desired. Hence, a peripheral circuit that can read minute changes in resistance must be used because the source-drain resistance of the transistor changes but a little. The use of the peripheral circuit is not more advantageous than to read the change in resistance of the MTJ cell.
As indicated above, a memory cell having a two-layered gate is disadvantageous in that the insulating film surrounding the floating gate cannot be as thin. Inevitably, the memory cell cannot be made smaller or driven at a lower voltage. By contrast, a semiconductor memory cell that stores the data in the form of resistance needs a large peripheral circuit for detecting the resistance of the memory cell.